The invention relates to a self-oscillating pulse-width modulator and a method for adjusting a self-oscillating pulse-width modulator.
A self-oscillating pulse-width modulator has the aim of generating a signal of a particular frequency. In Kikkert, C. J. et al., “Asynchronous Delta Sigma Modulation”, IEEE Proceedings, Apr. 1975, pages 83 to 88, examples of self-oscillating pulse-width modulators are disclosed which are shown in FIGS. 1 and 2.
The self-oscillating modulator 1 shown in FIG. 1 has the form of a control loop and comprises an input E1, an output A1, a forward branch 2 connected between the input E1 and the output A1 and a feedback loop 3 which loops an output signal a1 present at the output A1 back to the input E1. The output signal a1 looped back is subtracted from an input signal e1 of the modulator 1, present at the input E1. The resultant difference signal w1 is supplied to the forward branch 2. The forward branch 2 comprises an integrator 4 and a comparator 5.
For the self-oscillating modulator 1 shown in FIG. 1 to oscillate by itself, the phase shift of the open control loop must be 180° at the frequency at which the modulator 1 is intended to oscillate. In the self-oscillating modulator 1 shown in FIG. 1, this is achieved by correspondingly adjusting a hysteresis of the comparator 5. A further self-oscillating pulse-width modulator comprising a comparator, the hysteresis of which is adjusted in suitable manner, is known from U.S. Pat. No. 6,297,693 B1.
The self-oscillating pulse-width modulator 21 shown in FIG. 2 also has the form of a control loop and comprises an input E2 and an output A2 between which a forward branch 22 is connected. The forward branch 22 comprises an integrator 24 followed by a comparator 25 which, however, does not have a hysteresis. The modulator 21 also comprises a feedback loop 23 which, however, comprises a delay element 26. Due to the delay element 26, an output signal a2 present at the output A2 is looped back delayed in time as signal r2 to the input E2 where it is subtracted from an input signal e2 present at the input E2. The time delay of the delay element 26 is selected in such a manner that the phase shift of the open control loop is 180° at the frequency at which the modulator 21 is intended to oscillate.
However, adjusting the hysteresis of the comparator 5 of the modulator 1 shown in FIG. 1 and adjusting the delay of the delay element 26 of the modulator 21 shown in FIG. 2 with sufficient precision is relatively difficult.